It’s possible, to do these things, but it means bonding and unbonding the Sub-Mini from one room to then add it to the other …and by doing that all Trueplay tuning settings (if used) are lost and have to be recreated on rebonding the Sub. It’s not a process that I, or other regular community users here, would recommend. You might be better off bonding the two Symfonisk speakers as rear surrounds instead to the Beam, together with the Sub-Mini and play full ‘music’ audio and TV surround sound (DD5.1/Atmos) audio to all.

Indeed it seems to be so restrictive that's what I feared

Thank you very much for your help !

Olivier 

It's not an arbitrary restriction.  When adding a Sub or surrounds, the radios are reconfigured to use a direct 5GHz connection between the soundbar and the Sub/surrounds.  This reconfiguration entails a bunch of handshakes, which is why it takes time and cannot be an instantaneous switch.  Same for switching back. 

It’s also mostly a one way connection, quite dissimilar from a standard Sonos speaker configuration. Very little information travels from the ‘child’ speaker back to the ‘parent’.